Device for producing laser light

ABSTRACT

In a device to generate laser light, with a laser-active main resonator ( 2 ) and a coupled non-linear resonator ( 6 ), each of the two resonators ( 2, 6 ) being delimited by mirrors ( 8, 4; 4, 10 ), and one mirror ( 4 ) of the main resonator ( 2 ) being identical to a mirror ( 4 ) of the coupled non-linear resonator ( 6 ), it is provided that the non-linear resonator ( 6 ) is designed for a power loss less than 63% of the power which is coupled via the main resonator ( 2 ) and the mirror which delimits the coupled non-linear resonator ( 6 ), and the laser-active main resonator is designed so that in laser operation it has a threshold pump power which is less than one fifth, preferably less than one tenth, of the pump power which is used and using which this resonator is excited.

The invention concerns a device to generate laser light, with alaser-active main resonator and a coupled non-linear resonator. Each ofthe two resonators is delimited by mirrors, and one mirror of the mainresonator is identical to a mirror of the coupled non-linear resonator.

In the case of a resonator of a device to generate laser light, severalmodes of laser oscillation usually occur. Particularly in the case ofpulsed lasers in the picosecond range, which because of the shortness ofthe wave train emit a very large wavelength spectrum, this can result ininstabilities, since with each pulse statistically a different amplitudedistribution with reference to the different modes can be received.

This can be avoided with various methods of mode locking. Mode lockingmeans that the longitudinal modes of a laser have a fixed phaserelationship, and are thus rigidly coupled to each with respect tophase. The effect of this is above all that the laser emits its energyin the form of ultra-short pulses. Depending on the laser material andthe type of mode locking, these ultra-short pulses have pulse durationsfrom about 100 ps to a few femtoseconds. The time gap from oneultra-short pulse to the next pulse corresponds to the resonator cycletime.

The various known methods can be divided into active and passivemethods. The invention is explained below exclusively on the basis ofpassive methods.

In the case of passive mode locking, a pulse which cycles in theresonator itself modulates its amplitude. Active components foramplitude or frequency modulation can then be omitted.

For passive mode locking, a non-linear element, the effect of which canbe most simply understood on the basis of the saturable absorption, isused which the absorption is saturated at high light intensities and thetransmission is thus increased compared with low light intensities. Inthis way preferably a state of higher light intensity is forced. This isachieved above all with the resulting shortening of a pulse, which isthus set as a reproducible physical state. The saturated absorption canbe implemented, for instance, using semiconductor components, in whichcase the optical excitation enables elecrtons to switch completely fromthe valence band to the conduction band with suitably high intensity.

However, what is also particularly important here is the non-linearphase shift resulting from the mutual effect of the laser light and amedium which has a non-linear index of refraction, particularly becauseof the Kerr effect. Self phase modulation of the light occurs if theKerr non-linearity causes a time-dependent phase shift, for instance inan optical fibre. Additionally, self-focusing of the laser light ispossible if the Kerr non-linearity causes a location-dependent phaseshift. Such behaviour can be described theoretically analogously to thesaturable absorber which is given above, by a so-called artificialsaturable absorber.

Physically, it makes a big difference whether the process is resonant ornon-resonant. Resonant processes run more slowly than non-resonant ones.The optical Kerr non-linearity effect which is given above as an exampleis a non-resonant process of only a few femtoseconds rise time. Thusfast modulation is possible, so that essentially no recovery time has tobe taken into account.

Because of the non-resonant mutual effect, mode locking is possible inparticular over a wide wavelength range. Above all in the case of theKerr process with an optical fibre, the absorption is also very low, andcan be increased as required by providing a suitable length of fibre. Inthis way, simple adjustment of the lasers by suitable choice of theparameters of the non-linear resonator becomes possible, so that modelocking is achievable with different laser systems and lasers withdifferent output data, particularly maximum output power, pulse durationand repetition rate of the pulses.

For such mode locking, an auxiliary resonator to which a non-linearresonator is coupled is usually used, as described, for instance, inU.S. Pat. No. 5,054,027. The coupling is usually via an interferometer.However, this causes mode locking to become expensive and liable tofaults. Above all, because of the necessary precise positioning of aninterferometer to fractions of a wavelength, specially stableconstruction is required.

However, the specified US patent specification also shows as analternative a simpler arrangement, in which the mirror of the mainresonator, which is opposite the mirror to extract the generated laserlight, is equal to one of the mirrors of the non-linear resonator. Totransmit the light from the main resonator into the non-linearresonator, this mirror is partially translucent, with a transmission of5-35%.

However, some tests have shown that with the specified values nopermanently stable mode operation was possible. This embodiment isprobably given in the US patent specification rather as an idea, thepractical implementation of which is probably not immediately possible.One therefore continues to depend on the known interferometric methods,with the stated disadvantages.

In this connection, constructions with Fabry-Perot resonators orMichelson interferometers are known. For the special configurations,reference is made to the extensive literature.

A further disadvantage, which limits these methods to systems withcomparatively low output power of less than 7 W, is the necessity ofcoupling the laser light into a single-mode optical fibre, which hasonly a short lifetime with high coupled powers.

On the other hand, there is a high requirement for mode-locked laserbeam sources, which supply ultra-short pulses of less than 20 ps intothe femtosecond range. As well as metrology for capturing very precisetime flows, above all projection of pictures, particularly videopictures, using lasers should be mentioned here. Above all, there is arequirement because it is desirable to make large picture televisiontechnology possible for the consumer field.

A laser supplies a sufficiently high light density for this purpose.However, irrespective of the imaging technology which is used, becauseof the high coherence of laser light undesired glitter effects occur.They can be effectively avoided by a large spectral width or a smallcoherence length. With ultra-short pulses, this property is given fromthe outset.

The following features are therefore extremely desirable for a lasersource which can be used in this way:

-   -   a high average power of a few watts or more;    -   a short pulse duration of the laser pulses, of less than 20 ps;    -   a high repetition rate of the laser pulses, of more than 40 MHz;    -   a good beam quality, with diffraction limited as far as        possible;    -   as simple and compact construction as possible;    -   use of a mode locking method which can be used for different        laser materials and emission wavelength ranges;    -   robustness against external interference and adjustments of the        parameters of the laser or resonator; and    -   mode locking made possible for different repetition rates,        without significant restriction.

Because of the above-mentioned disadvantages, it cannot be expected thatall these aims can be achieved simply. This invention was thereforebased on a significantly simpler problem, which fulfils the essentialrequirements for increased stability and robust construction.

The object of the invention is to create a device which has robustconstruction and can be operated stably over time, to generate a pulsedlaser beam using mode locking.

On the basis of the prior art as stated in the introduction, the objectis achieved in that the non-linear resonator is designed for a powerloss less than 63% of the power which is coupled via the main resonatorand the mirror which delimits the non-linear resonator. Additionally,the laser-active main resonator must be designed so that in laseroperation it has a threshold pump power which is less than one fifth,preferably less than one tenth, of the pump power which is used andusing which the main resonator is excited.

The ratio of the pump power in laser operation and the threshold pumppower is the threshold magnification. The threshold magnification can bedetermined very easily from the frequency of the relaxationoscillations, i.e. the frequency with which the output power oscillatesaround the equilibrium position, and the total losses of the mainresonator.

H. G. Danielmeyer, “Low-frequency dynamics of homogeneous four-level-cwlasers”, Journal of Applied Physics 41, (1970) 4014 applies:r=1+4n ² f _(R) ²τ_(sp) T _(R)/(2α_(r))where r is the threshold magnification, f_(R) is the frequency of therelaxation oscillations, 2α_(r) is the total losses in the mainresonator, T_(R) is the resonator cycle time, and τ_(sp) is thefluorescence lifetime of the upper laser level.

As explained above, tests with the construction which is known from U.S.Pat. No. 5,054,027, with a main resonator and a coupled non-linearresonator, each of the two resonators being delimited by mirrors and onemirror of the main resonator being identical to a mirror of the couplednon-linear resonator, have shown no success with reference to theachievable stability.

On the other hand, it would be expected that this construction wouldallow a higher mechanical stability than with the known constructionswith interferometers, with which in any case a positioning precision offractions of wavelengths is required.

However, it has been found unexpectedly with a special test constructionthat even if the transmission of the common mirror of the main resonatorand non-linear resonator was less than 5%, and therefore quite differentfrom the teaching of the US patent specification, stable mode-lockedlaser operation became possible. Further consideration and tests, as isshown in more detail below, have led to the conclusion that theessential feature which allows robust construction with stablemode-locked laser operation is not the transmission of the mirror butexclusively conformity to the specified upper limit of 63% for the powerloss in the non-linear resonator, and that the laser-active mainresonator in laser operation has a threshold pump power which is lessthan one fifth, preferably less than one tenth, of the pump power whichis used and using which the laser-active main resonator is excited.

It was also completely unexpected that the above-mentioned desirableproperties were achieved completely unproblematically on the basis ofthe invention, and in particular even including the extensions which arepresented below.

In an advantageous extension of the invention, it is provided that themirror which delimits both the main resonator and the non-linearresonator has a transmission which is less than half, and in particularless than ⅓, of the transmission of the extraction mirror of the mainresonator.

The basis for this extension is that as little power as possible shouldbe coupled into the non-linear resonator. If the non-linear medium inthe resonator is, for instance, an optical fibre, this could bedestroyed by too much power being coupled into this medium. This thenrestricts the design of the laser for high average output power.

As can be seen below from calculations which are explained in detail,the stability criterion according to the invention is almost independentof the transmission of the medium between the main resonator and thenon-linear resonator. On the basis of the invention, therefore, thepower in the non-linear resonator can be kept almost arbitrarily small,which in turn has the consequence that the average output power can beincreased because of the lower load on the non-linear medium.

In this connection, it is particularly advantageous if the mirror whichdelimits both the main resonator and the non-linear resonator has atransmission less than or equal to 5%. This is a result which could notbe expected at all on the basis of the prior art quoted above.

According to another preferred extension, it is provided that thenon-linear resonator has a medium with a non-linear index of refraction.According to the invention it was provided, as shown, that the losses inthe non-linear resonator should be as small as possible. For thispurpose it is more advantageous to use a physical process which is notessentially based on absorption, such as, in particular, mode lockingvia the non-linear index of refraction.

In a preferred extension, it is provided that the medium with non-linearindex of refraction is at least partly in the form of the core of anoptical fibre.

In this connection, the advantages of an optical fibre have beenexplained above in more detail. However, what should be emphasised inparticular here is the improved capability of the medium for adjustmentto the predetermined conditions of the laser with an optical fibre,since the phase modulation and power can be set separately via thechoice of the length and core material and/or doping.

Above all, the losses are reduced in a preferred extension of theinvention, in which at least one end of the optical fibre has ananti-reflection coating.

In another preferred extension of the invention, it is provided that amirror which delimits the non-linear resonator is in the form of amirrored end surface of the optical fibre. In this way the constructionbecomes significantly simpler and less critical. In another arrangement,with a large mirror of normal size, it would have been necessary toprovide a lens to focus on the core, which would require firstly anadditional component and secondly a certain precision of adjustment ofthe focus on the core. This precision could be destroyed by an impact onthe laser if special actions were not taken to prevent it. Such impactscannot be excluded if a laser of the stated type is used in a largeprojection television set at home or if it is used commercially andtransported from event to event.

In the following advantageous extension of the invention, a selection ismade with respect to the mirror which is created by mirroring theoptical fibre end. This is characterized in that only the mirror whichdelimits the non-linear resonator and is opposite the mirror which isidentical to that of the main resonator is in the form of a mirrored endsurface of the optical fibre.

Because the optical fibre is only mirrored on one side and that side ofthe fibre which faces the main resonator remains free, theabove-mentioned advantages which result from the mirroring of theoptical fibre end are obtained, but at the other end lenses, opticalsystems or similar can be housed, allowing better coupling into theoptical fibre, to reduce the losses below the limit which is providedaccording to the invention.

In an advantageous extension of the invention, a lens for coupling lightinto the optical fibre is provided, and one side of it is mirrored as amirror for the main resonator. This not only saves components, but alsoincreases the stability, since the gap from mirror to lens automaticallyremains constant.

In a preferred extension of the invention, it is provided that a mirrorwhich delimits the non-linear resonator, and is not identical to themirror of the main resonator, has a surface which is curved concavely inthe direction of the interior of the resonator, and the focal distanceof which is in particular less than three times and in particular lessthan twice the resonator length of the non-linear resonator.

In experiments, it was established that in this way the stability of theintensity of the laser light could be significantly increased. Thiswould not have been expected from the outset, because the testconstruction also had a lens between the mirror and the optical fibre.It would have been expected that instead of the stated concave mirror aflat mirror could be used, if the focal distance of the otherwiseconcave mirror surface had been taken into account in the case of thatof the lens. However, this assumption was not confirmed experimentally.On the contrary, with a concave mirror surface it has been found thatthe precision of adjustment was improved significantly, which then has afavourable effect on stability.

Another advantageous extension of the invention is characterized in thatbetween the outer mirrors of the main resonator or non-linear resonatorfurther mirrors are provided to fold the light path, at least one ofthem being provided to couple in the pump light. With this extension,above all, the compactness and simplicity of the construction areimproved.

According to another preferred extension, it is provided that thelaser-active main resonator is in a form with a lasing medium betweenthe mirrors as a laser, for which two optical elements, particularly twomirrors with a curvature and a gap to the lasing medium are provided, onwhich basis the laser radiation can be emitted with a diffractionmeasuring number M²<2. Such optical elements can be mirrors or lenses.The definition of the diffraction measuring number not only allowsoutput beams of low divergence, but also makes specially good couplinginto the non-linear medium possible. In this connection, the upper limitof 2 has been shown to be specially favourable. Additionally, providingtwo mirrors to achieve a low diffraction measuring number is a speciallysimple action with which robust construction can easily be achieved.This teaching differs from known actions in the case of high-poweredlasers, where large diffraction measuring numbers are implemented.

Further advantages and special features of the invention are given bythe following presentation of embodiments with reference to the attacheddrawings.

FIG. 1 shows a block diagram to explain the principle on which theinvention is based;

FIG. 2 shows a schematic drawing for the practical construction of anembodiment;

FIG. 3 shows measured values for a pulse duration depending on a mirrorposition;

FIG. 4 shows stability measurements on an Nd:YVO₄ laser with a change ofthe resonator length;

FIGS. 5 a, b, c show various stability measurements for different lossesin the non-linear resonator.

The following embodiments concern devices with which mode lockingmethods are used.

The lasers which are shown are designed for ultra-short pulses withsimultaneously high average power and high repetition rate. The outputpower and pulse duration can essentially be adjusted and optimisedindependently of each other.

With the mode locking method which is used, the stability of the laseris also sufficiently high, so that the laser system does not have to beactively stabilised for length.

In the schematic example of FIG. 1, a main resonator 2, which isconnected via a mirror 4 of low transmission to a non-linear resonator6, can be seen. With the mirrors 8 and 10, in connection with the mirror4 in each case, two Fabry-Perot resonators are constructed. The mirror 8is also designed as an extraction mirror with a suitably chosentransmission. Contrary to other known embodiments of “APM” (additivepulse mode locking) lasers, in the case of the device of FIG. 1 theextraction from the main resonator is distributed to multiple extractionmirrors 8 and 4. The degree of extraction of these extraction mirrorscan be adjusted independently of each other, so that it is possible evenwith constant pump power to optimise the output power and the pulseduration of the laser.

By changing the degree of extraction of mirror 8, the usable outputpower can be varied. By choosing the degree of extraction of mirror 4,the power which is injected into the coupled non-linear resonator isadjusted. Since the resulting pulse duration depends on the power of theradiation in the coupled resonator, the pulse duration can be varied bychanging the degree of extraction of mirror 4, i.e. changing the qualityof the coupled resonator. Choosing two extraction mirrors simplifies theresonator construction further, since components such as a half-waveplate and a polarisation beam divider, such as are known from the priorart for coupling non-linear resonators, are also omitted. Additionally,the mode locking mechanism for devices such as are shown schematicallyin FIG. 1 is completely independent of the polarisation of the laserradiation.

The high passive stability of this method is achieved by minimising thelosses in the non-linear resonator. In all previously knownconstructions, the coupled resonator had low quality with high losses.

On the other hand, with the mode locking method which is used in FIG. 1,the losses in the coupled resonator are reduced to a minimum. This meansthat the transmission of mirror 4 can be chosen to be sufficientlysmall, in particular typically less than 5%. It is thus possible toachieve that even with length changes of several micrometers, typically4 to 6 micrometers, stable pulse operation is possible. However, if thetransmission of mirror 4 is too low, ultra-short pulses no longer occur,because then the power which is coupled back into the laser becomes toolow.

If the phase position of the resonators relative to each other ischanged, the high quality of the coupled non-linear resonator 6 resultsin strong intensity fluctuations in the coupled resonator. The intensityfluctuations cause a large change of the self phase modulation, i.e. thenon-linear phase in the coupled resonator. This change of the non-linearphase can compensate for the change to the linear phase within a largerange of several n, so that design interference in the main resonator isstill ensured. For this reason, with the construction which is shown andthe mode locking method which is used, stable mode-locked operation ofthe laser is possible without active stabilisation.

To characterise the losses in an optical resonator of length L, thefollowing three magnitudes are useful:

-   -   the fineness F,    -   the loss coefficient α_(r), and    -   the photon lifetime τ_(p)=1/c·α_(r)).

The fineness F is defined as usual as the ratio of the frequencyinterval to the spectral width of the modes.

The losses in optical resonators are essentially caused by absorption,scattering in the optical components and losses because of partiallyreflecting mirrors. In the coupled non-linear resonator 6, further losssources such as uncoated fibre end surfaces and losses which result fromimperfect mode adjustment of the laser light to the mode of thewaveguide of the optical fibre can be added.

Losses V_(F) because of the optical fibre can be described by thetransmission at the single pass. The transmission T_(F) is the ratio ofthe laser power before the fibre to the laser power after the fibre,i.e. the losses can be expressed by V_(F)=1−T_(F). In the experimentallyoperated embodiments, the transmission T_(F) was typically around 75% to80%. The losses V_(F) because of the optical fibre at a single pass werecorrespondingly 20% to 25%.

On a single cycle through the resonator of length L, the intensity ofthe wave is reduced by the factor (1−R_(TOTAL)), where:R _(total)=exp (−2·α_(r) ·L)

The total loss coefficient α_(r) is therefore:$\alpha_{r} = {{\frac{1}{2\quad L}\ln\quad\left( \frac{1}{R_{total}} \right)} = {\frac{1}{2L}{\ln\left( \frac{1}{1 - V_{total}} \right)}}}$where R_(total)+V_(total)=1, if V_(total) is the total loss factor of acycle.

To ensure the mechanism which is used here to compensate for a change ofthe linear phase by the non-linear phase, the light must still be ableto interact with itself after one cycle in the coupled resonator, i.e.it must not be weakened too much by the losses in the coupled resonator.The condition is thus that the photon lifetime τ_(p)=1/(c·α_(r)) must begreater than the resonator cycle time τ_(r)=2L/c. The following thenapplies:τ_(p)τ_(r)

If the above conditions are inserted into the inequality, the result is:1/1n(1/R _(total))<1R _(total)>1/eV _(total)<(1−1/e)≈0.63

The total losses in the coupled resonator should therefore be less than63%, to make the desired stable mode-locked operation of the laserpossible passively, without active stabilisation. This agrees withexperimental results, as is demonstrated later.

To achieve this high quality in the coupled resonator, further actionscan be taken in addition to the choice of a high reflectance of theextraction mirror 4. For instance, the coupling efficiency of the laserradiation into the medium 12 can, if the medium 12 is an optical fibre,be maximised by mode adjustment of the laser mode to the mode of thewaveguide by suitable choice of a lens, which focuses the radiation intothe optical fibre.

Such mode adjustment should also be carried out for the radiation whichis fed back from the highly reflective mirror 10 of the non-linearresonator 6 into the stated optical fibre for, for instance, thenon-linear medium. In particular, it has been shown that specially goodmode adjustment and thus optimisation of the coupling efficiency andquality of the resonator are achieved if, instead of a plane highlyreflective mirror 10, a mirror with a plano-concave surface and a focaldistance which is greater than the resonator length of the non-linearresonator 6 is used. In particular, results with a radius of curvatureof R=−2 m with a resonator length of approximately two metres have showngood results. In particular, the adjustment sensitivity of this mirror10 is then significantly reduced. Of course, as an alternative to aseparate mirror 10, the end surface of an optical fibre which is used asa non-linear medium can be mirrored.

If the construction of FIG. 1 is compared with those which are knownfrom the prior art, it can be seen that the number of components issignificantly reduced with the mode locking method which is used.Additionally, this type of mode locking ensures stable operation whichdepends little on the settings of the laser and resonator parameters. Inspite of the interferometric construction, mode-locked operation isinsensitive to mechanical faults.

A construction to generate continuously mode-locked coherent radiationis presented in more detail on the basis of a device according to FIG.2, which was constructed in the laboratory and on which measurementswere carried out. The mirrors 8, 4 and 10 have the same meanings as inFIG. 1.

The laser crystal 14 in the main resonator 2 consisted of Nd:YVO₄. Themain resonator was constructed with the mirrors 8, 20, 22, 24, 26 and 4.The pump radiation from laser diodes was coupled in according to thearrows which are shown above mirrors 22 and 24.

Mirrors 20 to 26 are highly reflective for the emission wavelength ofthe Nd:YVO₄ laser of 1064 nm. Mirrors 22 and 24 also had a hightransmission for light of the wavelength of the pump laser diodes of 808nm, and were given an anti-reflection coating for 808 nm on the back.

With mirrors 8 and 4, the extraction mirrors described above areimplemented with different transmission values for 1064 nm. Forinstance, mirror 8 had a transmission of 9% and mirror 4 had atransmission of 3%.

Both mirrors were given an anti-reflection coating for 1064 nm on theback, and were vacuum deposited onto a substrate with a wedge angle of0.5°, to prevent undesired feedback into the resonators. Likewise, theNd:YVO₄ crystal was given an anti-reflection coating for 1064 nm and 808nm.

The radii of curvature of mirrors 20 and 26 and the distances of theresonator mirrors 8 and 4 were chosen so that the radiation radius ofthe resonator mode in the Nd:YVO₄ crystal was adapted to the radiationradius of the pump radiation for an emission of the laser radiation witha diffraction measuring number M²<1.2. The resonator from mirror 8 tomirror 4 had a length of about 94 cm, so that a repetition rate of thelaser pulses of 160 MHz was achieved. Mirrors 20 and 26 had radii ofcurvature of −500 mm. All other resonator mirrors had plane surfaces andtherefore a radius of curvature of ∞.

The coupled non-linear resonator extends from mirror 4 via the deviatingmirrors 28 and 30 to mirror 10. In the coupled resonator, there is anoptical fibre as a non-linear medium 12 with the geometrical length of70 cm. The length of the coupled resonator thus corresponded to twicethe length of the main resonator, i.e. 1.88 m. The length is calculatedby the known method from the optical length of the optical fibre ofdimension 0.7 m×1.45, the index of refraction of n=1.45 and thegeometrical distances of the mirrors.

As the optical fibre, a polarisation-containing optical fibre with amode field diameter of 7.2 μm and a numerical aperture of NA=0.11 wasused. This fibre had a V number of 2.05 for λ=1064 nm, and thusrepresented a single-mode optical fibre for the wavelength of 1064 nm.The coupling of the laser light into the fibre and the collimation ofthe laser light behind the fibre were carried out with lenses 32 and 34.The lens 32 can also be, in particular, plane and mirrored on the sidewhich faces the main resonator 14, so that the mirrored surface acts asa resonator mirror instead of mirror 4.

After reflection at mirror 10, the laser light returns via components34, 12, 32, 30, 28 to the extraction mirror 4, and is coupled back intothe main resonator 2 again.

To adjust the length of the coupled resonator to the length of the mainresonator, mirror 10 was fixed to an X translation table, using which itwas possible to adjust the length with a precision of typically 10 μm.For fine adjustment of the resonator length, mirror 10 was additionallyfixed to a piezo-electrical adjustment element, using which theresonator length could be adjusted to a precision within nanometers.

The pump power through two laser diodes which were coupled in thecrystal 14 via mirrors 22, 24 had a value of 2×12 W=24 W. The laserthen, in operation without a coupled resonator, showed an output powerof 8.7 W behind mirror 8. With total losses in the main resonator of10%, the laser in laser operation had a threshold magnification of r34.6. The threshold pump power is thus 1/34.6 of the pump power which isused and with which this main resonator was excited. With a couplednon-linear resonator, the output power behind mirror 8 was 9.3 W higher.

This laser emitted ultra-short pulses of a pulse duration of 6.8 ps anda spectral width of 60 GHz. To determine the width, a sech²-shapedintensity course was assumed, a curve which fitted the observed pulsecourse excellently. The laser showed itself to be above all insensitiveto changes to the resonator length, which could be detuned continuouslyover a range of several micrometers, that is a phase change of severaln, without the mode-locked operation of the laser being interrupted.

The pulse duration of the laser pulses could be adjusted in particularby the change of the resonator length of the non-linear resonator 6. Forthis purpose, mirror 10 was displaced using the X translation table inthe direction of the resonator axis over a length range of almost 500μm. In FIG. 3, the pulse duration is shown as a function of themicrometer position, that is of the length detuning. It can clearly beseen that in spite of large length changes, the pulse duration changedonly in a range of 6.7 ps to 10.5 ps.

If the resonator is extended over the micrometer position of 5.26 mmwhich is applied in FIG. 3, the laser remains mode locked. However, anadditional narrow peak then appears in the spectrum, in addition to themode-locked spectrum. The laser then emits ultra-short pulses over acontinuous background. Shortening the coupled resonator under themicrometer position of 4.8 mm which is shown on the ordinate in FIG. 3resulted in emission of a sequence of mode-locked pulses, which wasadditionally low-frequency modulated, that is the so-called relaxationoscillation occurred. The frequency with this laser system was about 400kHz, with a repetition rate of 160 MHz.

In particular, as explained above, the long-term stability of the laseris particularly important. To determine it, the generated laser beam wasalso frequency doubled, and both the fundamental and thefrequency-doubled radiation of the laser were measured using aphotodiode. Because of the quadratic dependency of the peak power of thesecond harmonic, frequency-doubled radiation is specially sensitive tochanges of the pulse duration of the laser, so that the second harmonicreacts specially critically to instability.

All measurements showed stable long-term operation of this laser systemover three hours, in both the fundamental radiation and the secondharmonic, although the laser was not actively stabilised electronically.The output power of the laser showed a variance of 0.6% in thefundamental and 0.4% in the second harmonic.

For stability, the measurement which is shown in FIG. 4, of thedependency of the average power of the fundamental and the secondharmonic of the laser as a function of the resonator length detuning ofthe coupled resonator using a piezo adjustment element, is alsoimportant. In a test, a triangular voltage with a frequency of 0.02 Hzand a maximum range of 15V was applied to the piezo element (PI 840.10,15 μm/150V). Via this voltage, a maximum change of the resonator lengthof 2.25 μm was achieved. The bottom curve in FIG. 4 shows the voltageramp, the top curve shows the average output power, and the middle curveshows the second harmonic of the laser. It can clearly be seen that thesignal of the second harmonic hardly changes, in spite of the resonatorlength change by over 2 μm. The figure shows that with this constructionand the mode locking method which is used, stable mode-locked operationof the laser is possible without active (electronic) stabilisation.

In a further experiment, the output power of the laser was increased.For this purpose, the pump power was increased by choosing other, morepowerful laser diodes, and a different Nd:YVO₄ crystal was used. Thepump power was about 2×24 W=48 W. The output power in continuousoperation then rose to 16 W. With total losses in the main resonator of17%, this laser showed a threshold magnification of r=8.7. The thresholdpump power is thus 1/8.7 of the pump power which is used, and with whichthis main resonator was excited. The output power in mode-lockedoperation even rose to 18.3 W. Both powers were measured behind themirror 8 according to FIG. 2.

The laser continued to emit ultra-short pulses, with a pulse duration of6.7 ps and a spectral width of 60 GHz. The peak power of this laser wasthen 17.1 kW, and thus sufficiently large to be able to operateoptically non-linear frequency conversions in an optical parametricoscillator, or generation of higher harmonics, or total frequencymixing, with the highest efficiency.

The presented measurements show that with this construction and the modelocking method which is used, ultra-short pulses of very high averagepower can be generated without active stabilisation. Above all, with theincrease of the output power no deterioration of the stability of thelaser can be observed.

The laser emitted pulses of both high average power and short duration.With the mode locking method which is used, the output magnitudes of thelaser system can even be optimised independently of each other. As canbe seen from the data and the figures, the presented device is above allrobust against large changes of the parameters of the resonator and thepower.

To test the computer estimate which is explained above, a polarisationbeam divider and a λ/2 plate were inserted into the coupled resonator 6between lens 34 and mirror 10. Thus by turning the linear polarisationusing the λ/2 plate, part of the light could be extracted at thepolarisation beam divider, so that defined additional losses could beintroduced into the coupled resonator.

A stability measurement was then carried out for each of three differenttotal losses. For intensity measurement, the laser radiation was steeredonto a GaAsP photodiode, which is insensitive to the fundamentalradiation of the laser. However, a signal was always generated bytwo-photon absorption when the laser was mode locked, andcorrespondingly ultra-short pulses with high peak power were present.

The results of these experiments are shown in FIGS. 5 a-e. The breaks inthe signal which can be seen in FIGS. 5 a and 5 b, at a time ofapproximately 10 s, are solely the result of calibrating the zero pointby blocking the laser beam.

In FIG. 5 a, no additional losses were introduced. With a transmissionof the optical fibre in the single pass of T_(F=)78% and a degree ofreflection of mirror 4 of R=97%, the result for this experiment was atotal loss of V_(total)=41%. The total losses are thus less than isrequired according to the estimated condition of 63%. As can be seen inFIG. 5 a, the result is a constant signal of the diode. The laser showsstable mode-locked operation.

In FIG. 5 b, the total losses are V_(total)=59%. Additionally, 30% laserpower was extracted by an additional introduced beam divider. The totallosses of 59% are then near the estimated limit of 63%. The laser alsoworks significantly less stably, as can be seen in FIG. 5 b.

For the measurement which is shown in FIG. 5 e, the losses have beenincreased again to 68%. The total losses in this case are greater thanthe estimated limit. As the result, it can also be clearly seen in FIG.5 c that the laser now works very unstably. The laser is onlymode-locked in short intervals (maximum signal in FIG. 5 c), and emittedcontinuously between them (no signal in FIG. 5 c). The laser switchescontinually between these states. The operating state of FIG. 5 e thuscorresponds to such conditions as are known from previous publicationsand patent specifications for free-running, not actively stabilised APMlasers.

Indicating the limit of total losses in the coupled resonator accordingto the previous estimate is thus suitable for defining the mode lockingmethod which is used well.

In the embodiment of FIG. 2, the medium for the non-linear phase shiftis a polarisation-containing single-mode optical fibre of length L=70cm. However, it is also possible to use fibres of other lengths, withoutthe method of functioning of the device which is shown being affected.In the test construction described above, for instance, fibres of length40, 50, 60 and 100 cm were used, without having to accept disadvantages.Fibres with which the optical length of the coupled non-linear resonatorequals the length of the main resonator, that is of which the lengthdoes not correspond to an integer multiple of the latter, areparticularly advantageous. More compact, stable constructions can thenbe implemented. In relation to the embodiment of FIG. 2 (repetition rate160 MHz), this applies in particular to fibre lengths ≦50 cm.

To reduce the losses in the coupled resonator and increase the fineness,the fibre ends can also be given an anti-reflection coating.

Instead of the lens 34 and mirror 10 as in FIG. 2, the fibre end can bemirrored directly. In this way an even lower loss is obtained, and thefineness of the coupled resonator and thus the optical stability of thesystem are increased.

It is also possible to use fibres which do not contain polarisation, butthe system is then more sensitive to external disturbance, and thestability is less. The reason is changes of the polarisation direction,depending on external conditions of the fibre, such as their position orbending radius, after the pulse has passed through the fibre. Suchconditions can cause a modulation of the amplitude, above all becausethe Nd:YVO₄ crystal emits in polarised form, and therefore amplifies afixed polarisation direction preferentially.

In addition to the use of an optical fibre, waveguides on planarstructures, such as are used in integrated optics, can be used. Theresult is further applications of the method, because materials withgreater non-linear index of refraction can also be used. In comparisonwith an optical fibre, this would result in shorter lengths of thewaveguides, so that more compact systems and even higher repetitionrates become possible.

For high powers in the resonator, volume materials can also be used.Then, for instance, elements for coupling the laser light into thenon-linear medium 12, such as the lens 32, can be omitted, resulting insignificantly reduced losses. Media which work under the Brewster angleare particularly suitable for this. Additionally, the number of mediawhich can be used is increased, giving a further adjustment and/oroptimisation of the output data of the laser.

The method can be used even if amplification media other than Nd:YVO₄are used, because the pulse generation is caused by the non-resonant,non-linear medium, which is independent of the amplification medium. Asexamples, neodymium-doped crystals in various host lattices, such asNd:YLF, Nd:YAG, Nd:YVO₄, Nd:GVO₄, Nd:YPO₄, Nd:BEL, Nd:YALO, Nd:LSB,should be mentioned.

As well as the laser crystals which are doped with Nd, crystals whichare doped with, for instance, other rare earth ions can be used. Also,mode locking and pulse shortening should be possible in configurationswhich use other laser transitions in these laser media. As well as thelaser transition of the neodymium ion at 1.064 μm which is used in theembodiment, the known laser transitions around 1.3 μm and 900 nm canpreferably also be excited. Mode locking using laser crystals which aredoped with ions of the transition metals is also possible. Such lasercrystals, which can be tuned over a wider wavelength range, are forinstance Ti:sapphire, Cr:LiSaF and Cr:LiCaF.

The method is not restricted to a specific resonator configuration. Inparticular, there is also the possibility of achieving mode locking withresonators of greater or less length, i.e. lower or higher repetitionrate. A special advantage resulting from a higher repetition rate is,among other things, the smaller structural length of such a system.

In the embodiment, the ratio of the lengths of the main and coupledresonators was 1:2. Other integer length ratios of the two resonatorsare also possible. For instance, a ratio of 2:3 is suitable.

This method is also not limited to the longitudinal pump arrangement ofthe laser diodes which is shown in the embodiments. A transversearrangement of the laser diodes is also possible. The laser crystal isexcited laterally by the laser diodes. This transverse pumping isparticularly advantageous if the spatial beam quality of the laserdiodes which are used is low, as with high-powered laser diode bars orif the laser crystal has a large absorption length. Lamp-pumped systemsare also easily possible.

Increasing the power with this method is also possible. A configurationwhich, for instance, includes several laser crystals in the resonatorcan be chosen for this purpose. Higher pump powers of the laser diodesare also possible.

To shorten the pulses further, the known methods of compensation fordifferent group speeds within the pulse, known as GVD compensation, canbe applied. For this purpose, suitable optical components such ascompensation prisms, Gires-Tournois interferometers, compensationlattices or special dielectric laser mirrors can be inserted. Thesemirrors are known as “chirp mirror” or “GTI mirror”. With these mirrors,the different penetration depths of the various frequency portionswithin a pulse into the dielectric mirror layer result in the desiredcompensation of group speeds and thus a further reduction of the pulseduration.

Use of a non-linear resonator which is coupled in the way which ispresented here can also be combined with the previously known methods ofmode locking, e.g. acousto-optical and electro-optical modulation, oruse of a saturable semiconductor absorber. This contributes to a furthershortening of the generated pulses.

These explanations show that there are numerous possibilities foroptimising the presented devices for various fields of application andoutput parameters, such as power and pulse length. However, thediscussed changes always allow high robustness and stability, providedthat they conform to the criteria according to the invention.

1. Device to generate laser light comprising: a laser-active mainresonator and a coupled non-linear resonator, each of the two resonatorsbeing delimited by mirrors and one mirror of the main resonator beingidentical to a mirror of the coupled non-linear resonator, wherein thenon-linear resonator has a power loss less than 63% of the power whichis coupled via the main resonator and the mirror which delimits thecoupled non-linear resonator, and the laser-active main resonator has inlaser operation a threshold pump power which is less than one fifth ofthe used pump power by which this resonator is excited.
 2. Deviceaccording to claim 1, wherein the laser active main resonator has inlaser operation a threshold pump power which is less than one tenth ofthe used pump power by which the laser active main resonator is excited.3. Device according to claim 1, wherein the mirror which delimits boththe main resonator and the non-linear resonator has a transmission whichis less than half of the transmission of the extraction mirror of themain resonator.
 4. Device according to claim 1, wherein the mirror whichdelimits both the main resonator and the non-linear resonator has atransmission which is less than one third of the transmission of theextraction mirror of the main resonator.
 5. Device according to claim 1,wherein the mirror which delimits both the main resonator and thenon-linear resonator has a transmission less than or equal to 5%. 6.Device according to claim 1, wherein the non-linear resonator has amedium with a non-linear index of refraction.
 7. Device according toclaim 6, wherein the medium with non-linear index of refraction is atleast partly in the form of the core of an optical fibre.
 8. Deviceaccording to claim 7, wherein at least one end of the optical fibre hasan anti-reflections coating.
 9. Device according to claim 7, wherein amirror which delimits the non-linear resonator is in the form of amirrored end surface of the optical fibre.
 10. Device according to claim9, wherein the mirror which delimits the non-linear resonator and isopposite the mirror which is identical to that of the main resonator isin the form of a mirrored end surface of the optical fibre.
 11. Deviceaccording to claim 7, wherein a lens for coupling light into the opticalfibre is provided, and one side of it is mirrored as a mirror for. 12.Device according to claim 1, wherein a mirror which delimits thenon-linear resonator, and is not identical to the mirror of the mainresonator has a surface which is curved concavely in the direction ofthe interior of the resonator, and the focal distance of which is lessthan three times the resonator length of the non-linear resonator. 13.Device according to claim 1, wherein a mirror which delimits thenon-linear resonator and is not identical to the mirror of the mainresonator has a surface which is curved concavely in the direction ofthe interior of the resonator and the focal distance of which is lessthan twice the resonator length of the non-linear resonator.
 14. Deviceaccording to claim 1, further comprising: further mirrors provided tofold the light path, at least one of the further mirrors being providedto couple in the pump light, said further mirrors being positionedbetween the outer mirrors of the main resonator or the non-linearresonator.
 15. Device according to claim 1, wherein main resonator is ina form with a lasing medium between the mirrors as a laser, for whichtwo optical elements with a curvature and a gap to the lasing medium areprovided, on which basis the laser radiation can be emitted with adiffraction measuring number M²<2.
 16. Device according to claim 1,wherein the main resonator is in a form with a lasing medium between themirrors as a laser, for which two mirrors with a curvature and a gap tothe lasing medium are provided, on which basis the laser radiation canbe emitted with a diffraction measuring number M²<2.
 17. Deviceaccording to claim 3, wherein the mirror which delimits both the mainresonator and the non-linear resonator has a transmission less than orequal to 5%.
 18. Device according to claim 5, wherein the non-linearresonator has a medium with a non-linear index of refraction.
 19. Deviceaccording to claim 8, wherein a mirror which delimits the non-linearresonator is in the form of a mirrored end surface of the optical fibre.20. Device according to claim 10, wherein a lens for coupling light intothe optical fibre is provided, and one side of it is mirrored as amirror for the main resonator.
 21. Device according to claim 11, whereina mirror which delimits the non-linear resonator, and is not identicalto the mirror of the main resonator, has a surface which is curvedconcavely in the direction of the interior of the resonator, and thefocal distance of which is less than three times the resonator length ofthe non-linear resonator.
 22. Device according to claim 11, wherein amirror which delimits the non-linear resonator, and is not identical tothe mirror of the main resonator, has a surface which is curvedconcavely in the direction of the interior of the resonator, and thefocal distance of which is less than twice the resonator length of thenon-linear resonator.
 23. Device according to claim 12, furthercomprising: further mirrors provided to fold the light path, at leastone of the further mirrors being provided to couple in the pump light,said further mirrors being position between the outer mirrors of themain resonator or non-linear resonator.
 24. Device according to claim14, wherein the main resonator is in a form with a lasing medium betweenthe mirrors as a laser, for which two optical elements with a curvatureand a gap to the lasing medium are provided, on which basis the laserradiation can be emitted with a diffraction measuring number M²<2. 25.Device according to claim 14, wherein the main resonator is in a formwith a lasing medium between the mirrors as a laser, for which twomirrors with a curvature and a gap to the lasing medium are provided, onwhich basis the laser radiation can be emitted with a diffractionmeasuring number M²<2.